One of the forerunning types of exhaust gas recirculation systems for automotive internal combustion engines uses a microcomputer for the control of the exhaust gas recirculation rate. In an exhaust gas recirculation system of this type, the exhaust gas recirculation rate is varied through detection of the revolution speed of the engine and the load on the engine. The detected engine revolution speed and engine load are converted into electric signals as variable input signals for the microcomputer. In response to these signals, the microcomputer selects out of the table data preliminarily stored therein an optimum exhaust gas recirculation rate for the engine revolution speed and engine load represented by the signals.
During constant-speed operating conditions of an internal combustion engine, the relationship between the revolution speed of the engine and the load applied to the engine varies curvilinearly from a point corresponding to an idling condition of the engine, describing a curve which is commonly called the road-load curve and which is contained in a certain region in the load-revolution speed characteristics of the engine. Under accelerating and decelerating conditions of the engine, the load-revolution speed characteristics of the engine respectively vary within regions which are separate from each other across the constant-speed region.
As is well known in the art, furthermore, internal combustion engines tend to produce more nitrogen oxides during acceleration of the engine than under other operating conditions of the engine. For the reduction of the concentration of nitrogen oxides to be emitted from an automotive internal combustion engine, it is advantageous to recirculate more exhaust gases under accelerating conditions of the engine than during other modes of operation of the engine.
In a conventional computer-controlled exhaust gas recirculation system, the exhaust gas recirculation rate is controlled in such a manner as to be increased and decreased in response to the acceleration and constant speed regions, respectively, of the load-revolution speed characteristics of the engine.
During acceleration on a level road surface, the load-revolution characteristics of the engine follow the above mentioned road-load curve and are thus shifted from the acceleration region to the constant-speed region of the characteristics in a comparatively short while. During hill-climbing of the vehicle, on the other hand, the load-revolution speed characteristics of the engine stay within the acceleration region thereof. Under these conditions, the engine tends to overheat and would therefore cause troubles if exhaust gases are recirculated at rates which are optimum for the accelerating conditions of the engine. There are various other reasons for which it is desired to reduce the exhaust recirculation rate during hill-climbing of a vehicle as compared with the recirculation rate desired for the accelerating conditions of the engine, as will be discussed later.
The present invention contemplates provision of an exhaust gas recirculation system featuring control means suitable for eliminating these drawbacks which have thus far beed inherent in conventional computer-controlled exchaust gas recirculation systems for automotive internal combustion engines.